1. Field of the Invention
The present invention relates to a hard coating film to be coated on the surface of a member such as a cutting tool, a sliding member, or a die, and a manufacturing method thereof. More particularly, it relates to a hard coating film including a B—C system coating film or a cubic boron nitride film as the outermost surface layer, which is a hard coating film capable of being formed with good adhesion on the surface of the substrate of a cemented carbide, a high-speed tool steel, or the like, and a useful method for manufacturing such a hard coating film.
2. Description of the Related Art
A B- and C-containing coating film (hereinafter, may be referred to as a “B—C system coating film”) has a high hardness (a Vickers hardness HV of about 3500), and is also excellent in heat resistance, as typified by boron carbide (B4C). For this reason, the coating film has been studied to be applied to a cutting tool, a sliding member for use in a high-temperature environment, a die, or the like.
However, the B—C system coating film of boron carbide or the like is poor in adhesion to a cemented carbide (WC base cemented carbide) or a high-speed steel generally used as a material for a cutting tool or the like. For example, when the surface of such a material is coated with boron carbide, and the resulting coated material is applied to a cutting tool, coating film peeling unfavorably occurs at an early stage during use of the tool. Under such circumstances, various techniques have been proposed from the viewpoint of improving the adhesion to a cemented carbide or a high-speed steel.
As one of the techniques, a technique has been proposed in which, for example, a high-speed steel is used as a substrate, and a Ti layer is formed as an interlayer on the substrate surface, and then a B—C system coating film is formed on the interlayer by a sputtering process using a B4C target (see, e.g., Hauzer Techno Coating News Letter 17-4-2001). With this technique, Ti is formed as an interlayer, so that a Ti—B bond is formed between the B—C system coating film and the Ti layer, which enhances the adhesion of the B—C system coating film.
With the development of such a technique, it has been possible to more improve the adhesion than in the case where the B—C system coating film has been directly formed on the high-speed steel surface. However, the Ti—B bond involved in the adhesion is formed at only a part of the interface. As a result, it cannot be said that an adhesion sufficient enough for being applicable to a material for a cutting tool or the like has been achieved.
On the other hand, for the formation of a hard layer (external layer) of BxC (x=3.5 to 4.5) on the cemented carbide base metal surface, a proposal has been also made on a coating film configuration in which a C-rich graded texture layer of BxC (x=0.5 to 2), or a layer comprising a nitride, a carbide, or a carbonitride of Ti, Zr, Hf, or the like is formed as an internal layer (interlayer) (e.g., JP-A Nos. 57604/1992 and 57605/1992).
However, also with these techniques, the bonding region of the interlayer and the BxC (x=3.5 to 4.5) hard layer is only a part of the interface. As a result, it cannot be said that a sufficient adhesion has been achieved.
As a still other film, a cubic boron nitride film (hereinafter, may be abbreviated as a “cBN film”) has a high hardness and is also excellent in heat resistance. For this reason, the film has been studied to be applied to a cutting tool, a sliding member for use in a high-temperature environment, a die, or the like.
However, the cBN film has a large film stress, which unfavorably results in poor adhesion to a cemented carbide (WC base cemented carbide) or a high-speed steel generally used as a material for a cutting tool or the like. For example, when the surface of such a material is coated with the cBN film, and the resulting coated material is applied to a cutting tool, coating film peeling unfavorably occurs at an early stage during use of the tool. Under such circumstances, various techniques have been proposed from the viewpoint of improving the adhesion to a cemented carbide or a high-speed steel.
From the viewpoint of relieving the film stress of the cBN film, a technique has been proposed in which the cBN coating film is allowed to contain therein one or more elements selected from the group consisting of the transition metals in Groups 4A, 5A, and 6A of the periodic table, Al, and Si (see, e.g., JP-A No. 167205/2002).
However, with this technique, it cannot be said that the film stress of the cBN film is sufficiently reduced. Accordingly, the adhesion remains as unsatisfactory as ever.
On the other hand, another technique has also been proposed, wherein an interlayer in which the content of B and N changes in a graded manner is formed on the substrate surface, and the interlayer is allowed to contain C therein, so that B—C and C—N bonds are precipitated in the interlayer, followed by a sputtering process using a B4C target, thereby to form a cBN system coating film on the interlayer (see, e.g., “Surf. Coat. Technol.”, K. Yamamoto et al., 142-144, 881 (2001)).
With this technique, by achieving a higher hardness of the interlayer, the adhesion to the substrate is enhanced. However, the formation of the B—C bond or the C—N bond alone is insufficient for increasing the hardness of the interlayer. Further, the C—N bond is instable at high temperatures, and hence it decomposes by the temperature increase during use. As a result, unfavorably, the effects as high as expected are not achieved.